This invention relates to the field of acoustic transducers that use piezoelectric elements installed on a mandrel as acoustic signal generators for use in downhole telemetry applications. More particularly, the invention relates to an improved transducer apparatus configuration and method of assembly allowing wider machining tolerances and easier assembly than an earlier version of acoustic transducer patented by the applicant herein.
Background technology underlying recent developments in acoustic telemetry is described in detail in U.S. Pat. No. 5,703,836, which is incorporated herein in its entirety. Also incorporated by reference in its entirety is the patent application Ser. No. 09/306,672 filed in the United States Patent and Trademark Office on the same day as the instant application. Briefly, however, real time or near-real time data acquisition is advantageous in assessing and optimizing performance of subterranean equipment, such as is used in gas and oil wells. Often, in situ use of commonly employed data acquisition instruments is impossible or impractical due to harsh conditions that exist downhole. Communications can be established by way of hardwire connections between downhole and surface elements, however, such connections have proven to be expensive and unreliable under certain conditions. Likewise, attempts to employ traditional radio communications have been largely unsuccessful due to large electromagnetic attenuation.
For these reasons and others, communications systems have been developed that use the drill string elements, themselves, as a wave guide for communications signals. An example of this is the acoustic transducer described in the '836 patent mentioned above. The transducer in that patent comprises a hollow unitary mandrel having a cylindrical recess formed in the outer wall of the mandrel within which recess is captured a stack of piezoelectric elements in a temperature compensated interference fit. The transducer assembly also includes a power source and a protective shell that covers the region of the mandrel and captures the piezoelectric elements. The mandrel can be adapted to connect to production tubing that serves as the waveguide between the transducer downhole and the surface. The transducer is further adapted to receive information from a downhole measurement device such as a pressure/temperature gage.
Acoustic transducers tools, such as the one disclosed in the '836 patent, employ stressed piezoelectric elements. Compression of the piezoelectric elements is desirable to protect the ceramics from tensile failure. In the '836 patent a stack of washer-shaped piezoelectric discs is positioned about a cylindrical recess formed in a hollow mandrel. The discs are securely retained by thermal-expansion compensating rings which are, in turn, secured by the edge of the recess into which the discs and compensating rings are positioned. In the version of the apparatus disclosed in the '836 patent, the necessary compressive stress is obtained as a consequence of the method of assembly described there. Specifically, according to that method, the piezoelectric discs and thermal-expansion compensating rings are provided as pairs of half-cylinders that are emplaced in the cylindrical recess of the mandrel. The positioning of the half-cylinders takes place after the mandrel has been heated to sufficient temperature so as to cause the mandrel (and consequently the cylindrical recess cut into the mandrel) to expand slightly. At that point, the halves of the transducer elements and temperature compensating rings are positioned in the expanded recess, and the mandrel is allowed to cool. As the cooling takes place the mandrel contracts and the piezoelectric elements are captured securely in an interference fit in the mandrel recess.
While it is useful in many instances, the method of assembly just described can, however, prove cumbersome and difficult under certain conditions. Therefore, an unmet need exists for a simplified transducer apparatus and a simpler assembly method.